1. Field of the Invention
The present invention relates to an optical communication system. More particularly, the present invention relates to a wireless visible light communication system.
2. Description of the Related Art
The term “optical communication” refers to a communication system for transmitting a data-modulated optical signal through an optical fiber. A benefit of optical communication is that it may be used to transmit a large amount of data at a high speed. Optical communication systems may be classified into a wavelength division multiplexing scheme, a time division multiplexing scheme, a sub-carrier multiplexing scheme, etc. according to an optical signal transmission scheme. The wavelength division multiplexing scheme uses rays of light having different wavelengths as carriers for modulating data.
The wavelength division multiplexing scheme is a type of optical communication method for transmitting optical signals. The optical signals are data modulated into channels (i.e., light) having different wavelengths, through optical lines or the like. Since the wavelength division multiplexing scheme can transmit a plurality of optical signals through a single optical line, the wavelength division multiplexing scheme is an optical communication method suitable for transmission of a large amount of data at a high speed. Moreover, an optical communication system employing the wavelength division multiplexing scheme can transmit different types of data (e.g., Internet data, synchronous optical network (SONET) data, asynchronous transfer mode (ATM) data, etc.) through one optical line.
The wavelength division multiplexing scheme makes it easier to select bands according to wavelengths as compared to the frequency modulation and time division schemes. Therefore, the wavelength division multiplexing scheme can also be applied to wireless optical communication systems. However, the wireless visible light communication system may be restricted to indoor use or ultra short range communication because the wireless visible light communication system uses light as a carrier.
The wireless visible light communication may use light sources capable of generating white light as optical transmitters, or may user light sources capable of generating light of a wavelength which is invisible to the human eye (e.g., infrared ray, etc.).
When the wireless visible light communication system is applied in an indoor environment, optical signals generated from the light sources may additionally function as illumination, and light sources of three colors (red, blue and green) may be used to respectively generate optical signals to be used as carriers so that the optical signals can be at substantially the same state as natural light.
FIG. 1 is a block diagram schematically illustrating a configuration of a conventional wireless visible light communication system. A wireless visible light communication system 100 is a type of optical communication system which uses optical signals data-modulated using light in a visible wavelength band as a carrier. The wireless visible light communication system 100 includes optical transmitters 111, 112 and 113 capable of generating optical signals 101, 102 and 103 having different colors, and optical receivers 121, 122 and 123 for detecting the optical signals 101, 102 and 103 generated from the optical transmitters 111 to 113.
Each of the optical transmitters 111 to 113 includes light sources capable of generating light having mutually different colors, and may either further include an external modulator for modulating data, or may be configured in such a manner as to directly modulate data using the light sources. Since the wireless visible light communication system uses light in a visible wavelength band, which is visible to the human eye, as a carrier, it is necessary to transmit optical signals in the natural light state so as to minimize fatigue of the user. Accordingly, in an exemplary implementation, the optical transmitters 111 to 113 may be configured with light sources capable of generating three primary colors (red, blue and green) which are mutually different.
The optical receivers 121 to 123 include photoelectric converters 121a to 123a for converting input optical signals at corresponding wavelengths into electrical signals, and data detectors 121b to 123b for detecting data from the electrical signals, respectively.
Photo diodes or phototransistors may be used as the photoelectric converters 121a to 123a. In order to detect an optical signal at a corresponding wavelength, a wavelength selective filter (or a band pass filter or optical filter) for selectively transmitting only the optical signal at a corresponding wavelength from one of the received optical signals may be installed at the front position through which the photoelectric converters 121a to 123a receive the optical signals.
FIG. 2 is a graph illustrating a spectrum of an optical wavelength band used in a conventional wireless visible light communication system. Referring to FIG. 2, the graph illustrates powers of rays of light according to colors (i.e. green, blue and red). Accordingly, it can be understood that rays of light having colors may have different powers depending on wavelengths.
Although the wireless visible light communication system using light in a visible wavelength band is mainly used in an indoor environment, the wireless visible light communication system has a problem in that it is difficult to produce and maintain light in a uniform state of natural light (white light) due to the characteristic of colored light having different powers depending on wavelengths.